a view of human development as an ongoing, ever-changing interaction between the physical and emotional being and between the person and every aspect of his or her environment, including the family and society

signifies developments over time that appear to persist, unchanging, from one age to the next. Parents might recognize the same personality traits in their grown children that they saw in them as infants

a time when a certain type of development is most likely to happen and happens most easily. If that development does not occur during that sensitive period, it could still occur later. For example, early childhood is considered a sensitive period for language learning.